Extraction of bitumen from oil sands

ABSTRACT

Carbon disulfide is used as a solvent to extract bitumen from oil sands in an anhydrous countercurrent flow process that is compatible with existing procedures for upgrading bitumen. The solution is then fractionated to separate the carbon disulfide from the bitumen product. The bitumen is then treated with hydrogen to decompose the remaining carbon disulfide. The process should provide high yields and meet environmental concerns.

FIELD OF THE INVENTION

A process is provided for the extraction of bitumen from oil sandsemploying carbon disulfide as a solvent. In the process, oil sands arecontacted with carbon disulfide to dissolve the hydrocarbon contained inthe sands. Next, the resulting solution of hydrocarbon is fractionatedto separate the carbon disulfide from the product, the bitumen istreated with hydrogen to decompose the remaining carbon disulfide.

BACKGROUND OF THE INVENTION

Oil sands are growing in importance as a source of petroleum. Oil sandsare found in various parts of the globe, but the most significantdeposits occur in northern Alberta, Canada, along the Athabasca River.The composition of oil sands is a mixture of quartz, clay, water andabout ten percent heavy oil with a consistency of tar and known in theindustry as bitumen.

The accepted practice for extracting bitumen from oil sands is to mixthe sands with hot water and caustic to form an oil emulsion that issiphoned off from the solids. The mineral tailings are discarded afterabout 95 percent of the oil has been recovered. The extracted oil isupgraded by one of two processes to produce a synthetic crude oil thatis suitable for refining at a later stage.

While current technology is workable, it has some drawbacks,particularly as practiced on a large scale. Water pollution is caused bythe discharge of substantial quantities of wastewater. The energyefficiency of the process is poor. Lastly, the required investment inplant and equipment is considerable.

The object of the present invention is to provide an improved bitumenextraction process which is more cost-effective, meets environmentalconcerns and provides a product of the highest quality. This object, aswell as other features and advantages of the present invention, will beapparent from the following description which is based on the singledrawing figure that is included.

Other applications of the present invention will become apparent tothose skilled in the art when the following description of the best modecontemplated for practicing the invention is read in conjunction withthe accompanying drawing.

SUMMARY OF THE DISCLOSURE

The present invention comprises three steps for the extraction ofbitumen from oil sands. First, the oil sands are mixed with carbondisulfide to dissolve the oil and extract it from the solid material.Second, the solution of oil is fractionated to recover the oil andseparate the carbon disulfide. Third, the recovered oil is treated withhydrogen to destroy vestiges of carbon disulfide dissolved in the oil.

The process is carried out for the most part under anhydrous conditions.In this manner, water pollution from the discharge is tailings isavoided. Additionally, the recovery of oil is enhanced. Finally, byrecycling carbon disulfide to the extraction steps, its consumption iskept to a minimum.

BRIEF SUMMARY OF THE DRAWING

The description herein makes reference to the accompanying drawingwherein like reference numerals refer to like parts throughout theseveral views and wherein:

FIG. 1 is a block diagram showing three steps of the process, includingextraction, fractionation, and hydrotreating.

DETAILED DESCRIPTION OF THE PROCESS

The oil contained in oil sands is a heavy, viscous hydrocarbon mixturenot unlike tar. With the nomenclature of bitumen, this oil containsmolecules with twenty or more carbon atoms. By contrast, light sweetcrude, the premium feed to refineries, is mostly made up of compoundswith five to twenty carbon atoms. Bitumen is further characterized byits content of aromatic compounds in addition to aliphatic hydrocarbons.Bitumen also contains substantial quantities of bound sulfur.

Given the nature of bitumen, this raw material presents difficultproblems in its recovery from oil sands. As already mentioned, the priorart depends on forming a water-oil suspension that is separated from thesolids by flotation. Alternatively, bitumen can be heated to a hightemperature, in excess of 538° C., to reduce its viscosity to a pointwhere it will flow. This approach is used for in-situ recovery of oilfrom oil sands that lie too deep in the ground to be dug up by stripmining.

For this invention, I use carbon disulfide as a solvent for the bitumen.Carbon disulfide is an excellent solvent for this purpose: it iscompletely miscible with hexane as well as xylene. Up to 20 gm. ofparaffin wax and as much as 40 gm. of naphthalene can be dissolved in100 gm. of carbon disulfide at 20° C.

The low viscosity of carbon disulfide is also an advantage. At 20° C.,its viscosity is 0.32 centipoises. This value compares with about 20,000centipoises and up for bitumen. The viscosities of solutions can bedetermined by experiment or calculated from standard formulas. Furtherenhancing its ability to extract bitumen, carbon disulfide can beemployed in countercurrent equipment.

The cost of carbon disulfide is a major concern even though the reuse ofsolvent is assumed. To mine substantial quantities of oil sandscost-effectively requires that the solvent used, be cheap. Fortunately,carbon disulfide can be synthesized from plentiful materials that arefound in the oil sands deposits. It can be produced in an electricfurnace from elemental sulfur and petroleum coke. Alternatively, it canbe formed from carbonyl sulfide, which in turn is made from sulfurdioxide and carbon monoxide.

The solution of bitumen in carbon disulfide is fractionated to recoverthe bitumen. This step is most easily accomplished by distillation.Bitumen has a high boiling point whereas carbon disulfide boils at46.25° C. under 1 atmosphere pressure. Notwithstanding the ease ofseparation, some residual carbon disulfide can be expected to remain inthe bitumen.

Because of tightened specifications for petroleum products, includinggasoline and diesel fuel, there is a need to reduce sulfur values to aminimum. Such a requirement can be met through hydrotreating. This stepentails the reaction of the bitumen stream with hydrogen at elevatedtemperatures, in the range of 200° C. to 300° C. A catalyst may or maynot be used. Cobalt compounds, including cobalt-molybdenum alloys, havebeen found to be effective in this application.

The reaction that takes place when carbon disulfide is created withhydrogen is shown by the following equation.

CS₂+4 H₂→CH₄+2 H₂S

where CS₂ is carbon disulfide, H₂ is hydrogen, CH₄ is methane, and H₂Sis hydrogen sulfide. The thermodynamics for this reaction is extremelyfavorable under operating conditions so that it goes to completion.

The hydrotreating step can be integrated into the upgrading of bitumen.Because bitumen is so viscous, it cannot be pumped or processed in itsexisting state. Therefore, one of two processes is generally employed toreduce its viscosity: coking and hydrotreating. Both measures can betaken. The result is a synthetic crude oil that is acceptable forfurther processing.

A better understanding of the present invention can be gained byreference to FIG. 1. Oil sands, the ore process in the process, and thecarbon disulfide are fed to extractor 1. The resulting solution ispassed to fractionator 2, while the tailings are discarded. Bitumen fromunit 2 is hydrotreated in upgrader 3 to produce synthetic crude oil. Thecarbon disulfide from the fractionator 2 is recycled to extractor 1.

Making improvements in the processing of oil sands is particularlyurgent because the industry is expanding at a breakneck pace. Practicesthat have been accepted in the past cannot deliver the results demandedby current conditions. The present invention is ideal for meeting thetremendous growth that is forecast.

EXAMPLE

The viscosity of a solution of bitumen in carbon disulfide wascalculated using the following expression:

log Φ=x _(A) log Φ_(A) +x _(B) log Φ_(B)

where Φ is fluidity, the reciprocal of the coefficient of viscosity, andx is the mole fraction.

For a solution in which the mole fraction of bitumen is 0.1, theviscosity equals 0.90 centipoises. This result compares with theviscosity of 1.0 centipoises for water.

1. A process for the extraction of bitumen from oil sands comprising thesteps of: dissolving the bitumen in carbon disulfide to produce asolution; fractionating the resulting solution to recover bitumen andcarbon disulfide; and hydrotreating the recovered bitumen to removeremnants of carbon disulfide.
 2. A process according to claim 1 in whichthe extraction step is achieved by countercurrent flow of carbondisulfide and oil sands.
 3. A process according to claim 1 in which thefractionation is achieved by distillation.
 4. A process according toclaim 1 in which hydrotreating is conducted in the presence of a cobaltcatalyst.